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ZnO heterostructure composites with enhanced photocatalytic performance
Accepted Manuscript Title: Fabrication of porous 3D flower-like Ag/ZnO heterostructure composites with enhanced photocatalytic performance Author: Yimai Liang Na Guo Linlin Li Ruiqing Li Guijuan Ji Shucai Gan PII: DOI: Reference:
S0169-4332(15)00141-5 http://dx.doi.org/doi:10.1016/j.apsusc.2015.01.116 APSUSC 29553
To appear in:
APSUSC
Received date: Revised date: Accepted date:
7-11-2014 12-1-2015 17-1-2015
Please cite this article as: Y. Liang, N. Guo, L. Li, R. Li, G. Ji, S. Gan, Fabrication of porous 3D flower-like Ag/ZnO heterostructure composites with enhanced photocatalytic performance, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.01.116 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of porous 3D flower-like Ag/ZnO heterostructure composites with enhanced photocatalytic performance 1
College of Chemistry, Jilin University, Changchun 130026, P.R. China
*Corresponding authors: Guijuan Ji; Shucai Gan
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E-mail address: [email protected] (G. Ji); [email protected] (S. Gan).
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Yimai Liang1, Na Guo1, Linlin Li1, Ruiqing Li1, Guijuan Ji1*, Shucai Gan1*
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M
an
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Tel: +86 431 88502259.
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Abstract Porous 3D flower-like Ag/ZnO heterostructural composites were fabricated by hydrothermal
and
photochemical
deposition
methods,
without
using
any
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pore-directing reagents and surfactants. The obtained samples were characterized by
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XRD, SEM, TEM, XPS, BJH, DRS, PL spectrum. The experiment results show that,
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the silver nanoparticles successfully load on the surface of assembled ZnO flowers. The TEM and SEM morphologies demonstrated unique porous 3D flower-like
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structure of Ag/ZnO. Such special structure makes larger surface area and more active sites exposed during the reaction, facilitating the transportation of reactants and
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products and increasing the reaction rate. The photocatalytic degradation experiments under UV irradiation using Rhodamine B (RhB) as a model dye were executed. The
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relative results demonstrate that the photocatalytic activity of Ag/ZnO is obviously improved compared with the pure ZnO and the commercial TiO2 (Degussa, P25), the
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AZ-15 sample has the highest photocatalytic activity. The Ag/ZnO heterostructure composites promoted the separation of photo-induced electrons and holes, which was proved by Photoluminescence spectra (PL). Keywords: ZnO, Ag/ZnO, porous, 3D heterostructure, Photocatalytic properties 1. Introduction
During recent decades, the environmental problem has become a block for economic development and human health. Choosing green materials to handle contaminants is particularly important. Semiconductors, especially metal-oxide semiconductors materials, have become more and more attractive due to unusual
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optical, electrical and catalytic properties [1, 2]. Their application in photocatalytic fields has received intensive attention because the technique can eliminate pollutant with a high efficiency without causing additional damage to the environment [3, 4].
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ZnO, as an important wide-direct band gap (3.37 eV) metal-oxide semiconductor, is
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promising photocatalyst owing to its high catalytic efficiency, broader ultraviolet (UV)
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light absorption, lower cost than TiO2, and environmental sustainability [5]. In addition, the ZnO photocatalyst can be easily manipulated with desirable
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microstructures, which are proved to be important factors affecting the photocatalytic activities. Especially, 3D porous structures of ZnO show superior photocatalytic
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properties as compared with various low-dimensional ZnO nanostructures. In terms of morphological structure, the use of 3D hierarchically assembled nanoflakes or
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nanoplatelets structures is likely to improve structural–mechanical stability and dispersity in comparison to 1D vertically aligned nanowires and 0D nanoparticles
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structures [6].
However, the rapid recombination of photo-induced hole–electron pairs has
become principle factor affecting photocatalytic efficiency of ZnO photocatalyst [7]. Some attempts use electron scavenging agents, such as noble metals [8-10], metal oxides [11, 12], or carbon materials [13, 14], to reduce electron–hole recombination rate. Among them, ZnO–noble metal type heterostructures are one of the most promising hybrid materials. In principle, noble metal can serve as a scavenger for photogenerated electrons, promoting interfacial electron–hole separation in the photocatalytic process, thereby increasing the number of “live” photocharges and
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improving the photocatalytic activity [15]. Ag is the cheapest noble metal, which should be beneficial to the practical application of Ag-based materials. In recent years, the 3D Ag/ZnO heterostructural composites with different morphologies have been
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successfully synthesized using different methods. For example, Han et al prepared
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Ag/ZnO flower heterostructures on indium doped tin oxide (ITO) glass via a
Ag/ZnO
heterostructural
composites
by
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photoreduction method [16]. Zhang et al reported the fabrication of flower-like hydrothermal
combine
with
an
microwave-assisted polyol method [17]. Although there are some reports about flower-like Ag/ZnO to be successfully synthesized, the above-cited methods require
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complex and expensive equipments, toxic reagents, severe synthesis conditions or high synthesis temperature. Thus, it is a key challenge to synthesize 3D flower-like
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Ag/ZnO with an environmentally friendly method. Moreover, to the best of our knowledge, there is few literature reported about the architectures of porous 3D
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flower-like Ag/ZnO composites.
In the present work, we have been successfully prepared porous 3D flower-like
Ag/ZnO by hydrothermal and photochemical depositing methods. In contrast to the conventional methods for preparing porous and flower-like materials, in this paper, no pore-directing reagents or surfactants are used during the fabrication of porous 3D Ag/ZnO structures, which can avoid material post-processing and suffering from contamination [18, 19]. The porous structure of the Ag/ZnO photocatalyst is helpful for light-harvesting and mass transportation of organic molecules in the photochemical reaction of organic degradation [20]. A systematic study is presented
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for detailed characterization of microstructure, phase, composition, and optical property of the samples. Finally, the photocatalytic activity of as-prepared composites is compared, and a possible mechanism of photocatalysis is also discussed and
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proposed.
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2. Experimental
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2.1. Materials
Zinc nitrate hexahydrate (Zn(NO3)2 6H2O), urea (CO(NH2)2) and silver nitrate
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(AgNO3) were purchased from Tianjin Chemical Reagent Research Company. All chemicals were of analytical grade and were used directly without further purification.
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Distilled water was used in all experiments.
2.2 Synthesis of porous 3D flower-like ZnO
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1mmol Zn(NO3)2 6H2O and 2 mmol urea were dissolved in 70 mL of distilled water. After being stirred for 0.5h, the mixture was then transferred to a 50 mL
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Teflon-line autoclave and sealed. The autoclave was heated to 95°C and maintained at this temperature for 6h. After naturally cooling down to room temperature, the resulting precipitates were collected by centrifugation, washed three times with ethanol and distilled water and then dried at 70°C in air for 12 h. The final porous flower-like ZnO were retrieved through a heat treatment of the precursors at 300°C in air for 2 h with a heating rate of 1 °C/min. 2.3. Fabrication of porous 3D flower-like Ag/ZnO heterostructure composites Porous
3D
flower-like
Ag/ZnO
composites
were
synthesized
through
photodeposition method [21, 22]. In detail, 0.3 g of the flower-like ZnO powder was
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dispersed in 300 mL AgNO3 aqueous solution, and stirring for 0.5 h to achieve the adsorption of Ag+ ions onto the surface of flower-like ZnO. The obtained suspension was irradiated for 2 h under radiation with continuously stirring by employing a
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high-pressure Hg UV lamp (GGZ175, 175 W) to reduce the adsorbed Ag+ to Ag
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nanoparticles, subsequently. Then, the precipitates were centrifuged, washed with
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distilled water to remove the residual Ag+. The products were dried in vacuum at 60°C for 12 h. A series of porous flower-like Ag/ZnO composites were obtained by a
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similar procedure, except for different amount of AgNO3. The molar ratio of AgNO3/Zn(NO3)2 was changed in the range from 0 to 20% and the products were
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denoted as ZnO, AZ-5, AZ-10, AZ-15, and AZ-20 respectively, in which the last
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2.4. Characterization
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number corresponds to the molar ratio of AgNO3/Zn(NO3)2.
X-ray diffraction (XRD) analysis of porous flower-like ZnO and Ag/ZnO were
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performed using a D/Max-IIIC (Rigaku, Japan) with Cu Ka radiation. The
morphology and dimension of the products were characterized by S-4800 field emission scanning electron microscope (FE-SEM, Hitachi, Japan) with an acceleration voltage of 3 kV. Further detailed structural analysis of individual particles was carried out by using a Hitachi 8100 transmission electron microscope (TEM, Hitachi, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI-5000CESCA system with Mg K radiation (hr = 1253.6 eV). The X-ray anode was run at 250 W, and the high voltage was kept at 14.0 kV with a detection angle at 540. All the binding energies were calibrated by using the
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containment carbon (C 1s = 284.6 eV). The Brunauer-Emmett-Teller (BET) surface area of the powders was analyzed by nitrogen adsorption on a Micromeritics ASAP 2020 nitrogen adsorption apparatus (U.S.A.). The sample was degassed in vacuum at
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150°C for 7h before nitrogen adsorption measurement. The BET surface area was
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determined by a multipoint BET method using the adsorption data in the relative
determine
the
pore size
distributions for
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pressure (P/P0) range of 0.01-0.99. Adsorption branches of the isotherms were used to the
samples
studied
via
the
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Barrett-Joyner-Halenda (BJH) method. The volume of nitrogen adsorbed at the relative pressure (P/Po) of 0.99 was used to determine the pore volume. Differential
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scanning calorimetry-thermogravimetric analyzer (DSC/TGA 1600 LF, METTLER TOLEDO, Switzerland) up to 800 °C was performed on the sample at a heating rate of
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10 °C min−1 while N2 gas flow rate of 60 ml min−1. 2.5 Photocatalytic experiments
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The photocatalytic performance of Ag/ZnO heterostructure was evaluated by using
RhB as presentative dye pollutant. A high-pressure Hg UV lamp (GGZ175, 175 W) with a maximum emission at 365 nm served as the UV light resource. A 500W Xenon lamp (CEL-HXF300) was used as the visible light source with a 400 nm cut-off filter. In a typical procedure, the 0.04 g catalyst (Ag/ZnO, Degussa P25 and ZnO) was dispersed into 20 mL aqueous solution of RhB (10 mg L-1). After stirring in the dark for 0.5 h, the suspensions were placed under light irradiation. During the photoreaction, the samples were collected at regular intervals and centrifuged to remove the catalyst. The concentration of target organic solution before and after
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degradation was measured by using a UV–vis spectrometer (UV-2550, Shimadzu, Japan). 3. Results and discussion
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3.1 Morphological and structural analysis
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The structural properties of the synthetic samples were analyzed by X-ray
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diffraction (XRD). Fig. 1 shows the XRD patterns for the porous flower-like ZnO and Ag/ZnO composites with different Ag content. Three main distinct peaks at 31.78°,
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34.42°, and 36.28° are observed in above patterns, which are indexed to the (100), (002), and (101) diffractions of the wurtzite ZnO (JCPDS no. 36-1451). From Fig.1, it
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can be seen that Ag/ZnO composite samples had four major peaks at 38.14°, 44.26°, 64.42°, 77.44°, which were readily assigned to the (111), (200), (220), and (311)
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planes of face-center-cubic (fcc) structure of silver (JCPDS no. 04-783), respectively. Additionally, no other peaks and remarkable shifts with increase amount of Ag, which
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indicates that no related solid solution of ZnO and Ag is formed or lattice expansion and shrinkage of Ag/ZnO should be negligible. In order to characterize the morphologies of the as-prepared ZnO and Ag/ZnO
composites, SEM and TEM were carried out. As is shown in Fig. 2a, ZnO has a three-dimension (3D) flower-like morphology and a few microns in size. A detailed observation of a single ZnO is shown Fig. 2b, those flower-like architectures are formed by a lot of self-assembled porous nanoplates and the thickness of most of nanoplates is 10-19nm. In addition, we can obviously observe that there are numerous pores in the nanosheets in the inset of 2b. Such a porous network structure can
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potentially increase the accessible surface area for enhanced reactant diffusivity, which is highly favourable for catalytic applications. Fig. 2c and d shows the SEM
morphology has not been changed upon modification with Ag.
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image of a typical Ag/ZnO flower-like structure. It can be seen that the ZnO
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Fig. 3a shows the TEM image of a typical Ag/ZnO flower-like structure,
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confirming the results obtained by SEM. It can be found that the Ag/ZnO particles remain flower-like structure and a great amount of spherical Ag particles with
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diameter ranging from 90 to 130 nm are observed on the surfaces of ZnO flowers, suggesting that the structure of ZnO is maintained after the formation of Ag
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nanoparticles. The porous feature of Ag/ZnO nanosheets is further confirmed by the corresponding TEM image (Fig. 3b) of individual ZnO porous nanosheets. It is
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apparent that the mesopores with irregular shape are of 10–50 nm in size. The large amount of the nanopores in the nanostrips should be beneficial to enhancing the
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specific surface area of the products. The EDS analysis, as seen in Fig. 3c, confirms that these nanoparticles are only composed of oxygen, zinc, and silver elements, which is consistent with the XRD results. 3.2 XPS analysis
To clarify the surface structures of porous flower-like Ag/ZnO composites, XPS
measurement was conducted on AZ-5. The survey result in Fig. 4a demonstrates all peaks are ascribed to Zn, Ag, O and C, confirming the heterostructure is composed of three elements of Zn, Ag and O. The results are in good agreement with XRD and EDS as described above. The presence of C is mainly from pump oil due to vacuum
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treatment before the XPS test [23]. High-resolution spectra of Ag, Zn and O species are shown in Fig. 4 b-d, respectively. From Fig. 4b, it can be seen that the binding energies of Ag 3d5/2 and Ag 3d3/2 for Ag/ZnO is at 367.3 and 373.4 eV, which shift
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remarkably to lower binding energy according to the corresponding values of the
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synthesized pure metallic Ag (the standard binding energies of Ag 3d5/2 and Ag 3d3/2
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for bulk Ag are about 368 and 374.2 eV, respectively) [24]. This phenomenon is similar to the results obtained from ZnO/Ag nanorods heterostructures and Ag/ZnO
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hollow sphere composites [25, 26]. The binding energy shift of Ag is mainly attributed to electron transfer from metallic Ag to ZnO crystals. Since the work
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function of silver is smaller than that of ZnO, once the Ag nanoparticle and ZnO attach together, electron transfer occurs from Ag to the conduction band (CB) of ZnO
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nanocrystal. Thus, a new Fermi energy level in Ag/ZnO is formed, leading to the higher valence of Ag [27]. Because of the lower binding energy of monovalent Ag
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than that of zero-valent Ag, the shift to lower binding energies of Ag 3d5/2 and Ag
3d3/2 further verifies formation of Ag/ZnO heterostructural composites [28]. The Zn 2p3/2 and Zn 2p1/2 peak of the Ag/ZnO sample, as shown in Fig. 4c, have a value of
about 1021.3 and 1044.3 eV, which are similar to that of pure ZnO, this finding confirms that Zn exists mainly in the Zn2+ chemical state on the sample surface [29]. In Fig. 4d, the O 1s profile of Ag/ZnO can be fitted into two peaks (530.0 and 531.8 eV), indicating the presence of two different kinds of O species in the sample. The peaks are associated with the lattice oxygen of ZnO and chemisorbed oxygen of the surface hydroxyls, respectively [30]. Herein, it is worth mentioning that the surface
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hydroxyls can produce primary active hydroxyl radicals, which are capable of trapping photoinduced electrons and holes. Thus, the surface hydroxyls are very important for photocatalysis [31].
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3.3 BET analysis
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The mesopores of porous flower-like Ag/ZnO composites are further confirmed
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by the responding plot of the pore size distribution is determined by using the Barrett-Joyner-Halenda (BJH) method from the desorption branch of the isotherm.
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Full nitrogen sorption isotherms of the porous flower-like ZnO and AZ-15 are shown in Fig. 5. It can be noted that most of the pores possess diameters in the range of
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10–50 nm with the peak zenith of ZnO and AZ-15 appearing around 22.3 and 17.8 nm (see the inset of Fig. 5). Pore size distribution curves confirm the mesoporous nature
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of porous flower-like ZnO and Ag/ZnO heterostructures. The specific surface area
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calculated by BET method is 49.0 m2 /g for porous flower-like ZnO and 34.5 m2/g for
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AZ-15. The results are consistent with a previous report [32]. Table 1 summarizes the calculating BET surface areas of porous flower-like ZnO and Ag/ZnO heterostructures and comparion with the relevant literature reports [33-36]. 3.4 UV–vis diffuse reflectance and Photoluminescence spectrum The UV–vis spectra of porous flower ZnO and Ag/ZnO composites with
variable contents are shown in Fig. 6. The absorption peaks at UV region are assigned to the absorption of ZnO. The pure ZnO maximum absorption edge is located at around 372 nm. As shown in Fig. 6b-d the absorption of the porous flower-like Ag/ZnO composites shows the enhanced absorption at UV region. By comparing ZnO
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with Ag/ZnO, it can still be found that the performance of UV–vis absorption is apparently enhanced as Ag is doped. In addition, there are two absorption band near 490 nm and 670 nm for all the Ag/ZnO catalysts besides the absorption edge of ZnO,
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which can be assigned to the characteristic absorption of surface plasmon resonance
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(SPR) caused by nanometallic Ag [37]. The surface plasmon resonance effect of Ag
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particles can increase the absorption capability for visible light, which induced the visible light responsive photocatalytic activity for ZnO photocatalyst [38]. Compared
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with the porous ZnO, the absorption spectrum of Ag/ZnO reveals an expansion from UV to visible area, which indicates potential application of porous 3D Ag/ZnO
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heterostructure composites as sunlight-derived photocatalyst.
Notably, it has been duly established that the photoluminescence spectra originate
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from the direct recombination of photoexcited electrons and holes [39]. The Photoluminescence (PL) spectra (excitation at 325 nm) of as-prepared ZnO and
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Ag/ZnO with different Ag contents composites are shown in Fig. 7. A UV peak centered at about 380 nm is observed, which is assigned to the recombination of electron–hole pairs in the semiconductor through an exciton–exciton collision process. Meanwhile, it is worth to note that the intensity of PL peak varies with the introduction of Ag, inferring that the Ag can change the recombination of electron–hole pairs. On the one hand, the PL spectra of Ag/ZnO shows a decrease in the intensity of UV emission peak, which results from electron transfer from ZnO to Ag on the photocatalyst interface. Moreover, it can be seen clearly that the sample AZ-15 shows the minimum PL intensity [40]. This phenomenon can be interpreted as
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that, with the increase of Ag, more metal sites are formed and available to trap electrons, leading to a corresponding increase in separation effects for the photoinduced electrons and holes, which would enhance the photocatalytic properties
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of the as-obtained heterostructures [41]. On the other hand, when the Ag content
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exceeds certain value (as sample AZ-20), the PL intensity increases again. This can be
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attributed to absorption or reflection of emission at the Ag/ZnO interface, which is mainly induced by the strong surface plasmon absorption of Ag particles [42].
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According to the above analysis, it is that Ag modification can make the intensity of PL decrease. The weaker is PL response, the better is the separation efficiency of
performance of Ag/ZnO composites.
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photoinduced electron–hole pairs, and the higher will be the photocatalytic
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3.5 Photocatalytic performance
The photocatalytic activity of Ag/ZnO composites was evaluated by taking RhB as
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the target dye pollutant. Under UV light irradiation, the photocatalytic results of RhB in the presence of different catalysts (Ag/ZnO, ZnO and P25) are all shown in Fig. 8. Under dark conditions, the concentration of RhB has no obvious change for long time in the presence of the catalysts. Similar result was obtained when reaction was performed under UV-light irradiation but without using any catalysts. The results indicate that both the light and ZnO are necessary for effective photodegradation of RhB dye. As shown in Fig. 5, the photodegradation efficiency of RhB is about 27%, 21%, 56%, 63% ,79% and 41% for P25, ZnO, AZ-5, AZ-10, AZ-15 and AZ-20, respectively, when the reaction was performed under UV-light for 120 min. It can be
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seen that the Ag/ZnO composite is a superior photocatalyst to Degussa P25 for photodegradation of the RhB. However, the pure ZnO particles exhibit a lower activity than that of Degussa P25. The photodegradation of RhB can be considered as
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a pseudo-first-order reaction, the rate expression is given by equation: ln(C/C0) = -kt
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[43]. The apparent rate constant k was listed in Table 2. Based on the results, one can
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see that k value for all Ag/ZnO samples is higher than Degussa P25 and pure ZnO. The AZ-15 sample shows the highest catalytic activity with rate constant k = 0.0365
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min−1 that is about 2 times higher than that of pure ZnO (0.00173 min−1). These results show that the photocatalytic activity of ZnO can be obviously improved in the
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presence of Ag NPs. They also indicate that an appreciable amount of Ag is necessary to enhance the photocatalytic performance.
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It can be seen that the photocatalytic activities of these samples vary in the following order: ZnO< P25< AZ-20< AZ-5 < AZ-10< AZ-15. We can note that the
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activity of Ag/ZnO composites firstly increased and then decreased with the increase of Ag amount in the composites. The AZ-15 exhibits the highest photodegradation rate. It consistent with the results of Fig.7, the PL studies indicate that the deposition of appropriate amount of Ag on ZnO surface is necessary to inhibit the recombination of photo-induced electron–hole pairs [44]. It is well known that the strong ultraviolet emission corresponds to the high electron–hole recombination. That is to say, the AZ-15 shows much higher electron–hole segregation efficiency than other as-synthesized catalysts, which is an important reason for its much better photocatalytic activity. However, the photocatalytical activity of Ag/ZnO remarkably
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decreases when the Ag/Zn atomic ratio is increased to 20%. This phenomenon can be interpreted as follows: (1) after the Ag content exceeds the optimum value, the active sites on the surface of Ag/ZnO will decrease and the UV light adsorption by ZnO can
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be partly shielded, decreasing the utilizing efficiency of photoelectrons. (2) Ag can
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also reversibly act as charge carrier recombination centers, which is caused by the
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electrostatic attraction of negatively charged Ag and positively charged holes when Ag is excessive [45]. The over accumulations of electrons on metal surface could
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attract the photogenerated holes to the metal sites, and promote the recombination of charge carriers [46]. In other words, the over accumulations of negatively charged Ag
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could increase the possibility of hole capture, reduce the efficiency of charge separation, and suppress the photocatalytic activity of Ag/ZnO.
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Based on above analysis, the effect of Ag modification on the photocatalytic performance ZnO could be explained by followed points: (1) Ag NPs could inhibit the
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recombination of photoinduced carriers of ZnO, thus the photocatalytic performance of ZnO could be improved by Ag modification; (2) excessive Ag can shield the UV light adsorption of ZnO, thus decrease the photon utilizing efficiency. As results, the photocatalytic activity of the catalyst is therefore depressed as more Ag was used. Present results about effect of Ag content on photocatalytic activity are consistent with previous reports [44, 47]. Further investigation on photocatalytic activity of porous 3D flower-like Ag/ZnO under visible light (λ>400 nm) was also carried out. Fig. 9 shows the
photodegradation curves of RhB by different catalysts under visible-light irradiation.
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It can be seen that the pure ZnO and P25 have weak visible light photocatalytic activity due to ZnO and TiO2 with wide band gap energy. Comparing with the pure ZnO, all Ag/ZnO samples exhibit much higher photocatalytic activities. As discussed
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in the diffuse reflectance spectroscopy (Section 3.4), pure ZnO has no absorption in
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the visible region, therefore, it has no photocatalytic activity under the visible light,
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but, by depositing Ag nanoparticles RhB photodegradation improved under similar condition. Under visible light irradiation, ZnO cannot be excited due to its wide
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bandgap energy (3.37 eV). However, due to the surface plasmon resonance of Ag nanoparticles, most photogenerated electrons may be excited from the surfaces of Ag
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nanoparticles to the conduction band of ZnO and then diffuse to the surrounding medium to promote photocatalysis. This provides a facile and efficient pathway for
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the photocatalytic degradation of RhB in the visible-light region [48]. 3.6 Photocatalytic mechanism
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Scheme diagram of the photocatalytic reaction on Ag/ZnO heterostructure is
presented in Fig. 10. Under irradiated under UV light, photon energy is higher than or equal to the band gap of ZnO crystals and electrons (e−) in the valence band (VB) will be excited to the CB with simultaneous generation of the same amount of holes (h+) in
the VB(Eq. (1) ). Due to the higher CB of ZnO than the new equilibrium Fermi energy level of Ag/ZnO, some electrons are injected into Ag nanoparticles quickly. At the same time, a Schottky barrier formed at the semiconductor–metal interface, and the holes can remain in the semiconductor surfaces [49]. Hence, silver, acting as electron sinks, not only reduce the recombination of photoinduced electrons and holes
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but also prolong the lifetime of photogenerated pairs. The electron will continually transfer until the overall Fermi level of the metal-modified ZnO system shifts toward more negative potential and ultimately equilibrates with that of ZnO. Subsequently,
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the transferred electrons can be easily trapped by the adsorbed O2 molecules to
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produce •O2- radical (Eq. (2) ). While, photoinduced holes in ZnO can be also readily
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scavenged by the immanence H2O molecules to yield •OH radicals (Eq. (3) ). The latter is an extremely strong oxidant for degeneration of organic chemicals, which can
follows [50]. ZnO h ZnO(e- h )
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-
h vb OH OH
(2)
(3)
d
-
e cb O 2 O 2
(1)
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be destroyed into CO2 and H2O activities (Eq. (4) ). This process can be proposed as
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4. Conclusions
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OH RhB degradation product (4)
In summary, we have been successfully fabricated porous 3D flower-like Ag/ZnO
by hydrothermal and photochemical depositing methods. Such special structure with an open and porous surface layer, which be benefit for facilitate the diffusion and mass transportation of organic molecules and oxygen species during the photochemical reaction. The photocatalytic degradation experiments demonstrate that, the as-prepared Ag/ZnO samples exhibited superior photocatalytic performance, which was attributed to the unique porous 3D structure of Ag/ZnO and effectively separation of photo-generated charge on flower-like ZnO by employing Ag
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nanoparticles as a conductor. In addition, the AZ-5 composite showed the highest photochemical activity. On the basis of the structural characterizations and photocatalytic results, the effect of Ag nanoparticles on the photocatalytic
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performance of porous 3D flower-like ZnO composites can be summarized as follows:
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(1) Ag deposit acted as electron sinks to enhance the separation of photoexcited
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electrons from holes; (2) Ag particles also act as recombination centers at high silver deposition which leads to the decrease of the material’s photocatalytic activity. The
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study revealed that the prepared composite exhibits a high potential for treatment of contaminated water applications.
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Acknowledgments
This work was supported by Mineral and Ore Resources Comprehensive Utilization
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of Advanced Technology Popularization and Practical Research (MORCUATPPR) founded by the China Geological Survey (grant no. 12120113088300). It was also
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supported by Key Technology and Equipment of Efficient Utilization of Oil Shale Resources (no. OSR-05) and the National Science and Technology Major Projects (no. 2008ZX05018). References
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Figure captions Fig.1 XRD patterns of (a) porous flower-like ZnO nanostructure, (b) AZ-5, (c) AZ-10, (d) AZ-15, (e) AZ-20.
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Fig. 2 SEM images of (a, b) the porous flower-like ZnO; (c, d) AZ-5 heterostructure
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composites.
spectrum of AZ-5 composites revealed by EDS.
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Fig. 3 TEM images of (a, b) porous flower-like Ag/ZnO composites; (c) the elemental
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Fig. 4 XPS spectra of the porous flower Ag/ZnO (a) XPS full spectra of the samples, (b) Ag 3d spectra,(c) Zn 2p spectra, (d) fit spectra of O 1s for Ag/ZnO.
distribution of ZnO and AZ-15.
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Fig. 5 N2 adsorption/desorption isotherms of ZnO and AZ-15 and BJH pore diameter
(d) AZ-10, (e) AZ-15.
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Fig. 6 UV–vis diffuse reflectance spectra of porous 3D (a) ZnO, (b) AZ-5, (c) AZ-20,
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Fig. 7 PL spectra of Ag/ZnO composites prepared with various Ag contents. Fig. 8 Photodegradation curves of RhB by different catalysts under UV-light
irradiation.
Fig. 9 Photodegradation curves of RhB by different catalysts under visible-light
irradiation.
Fig. 10 UV-light-induced charge separation mechanism of the porous flower-like Ag/ZnO and photocatalytic mechanism.
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Table 1 BET surface areas of porous flower-like ZnO and Ag/ZnO heterostructures
BET Surface area ± 0.1 (m2/g) 49.0
Porous flower-like Ag/ZnO
34.5
nanoplates ZnO [32]
15.5
0nanospheres ZnO [33]
20.8
Commercial ZnO powder [34]
3.6
Ag/ZnO particles [35]
3.5
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porous flower-like ZnO
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Photocatalyst
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prepared in this paper and relevant literature reports.
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Table 2 The apparent rate constant K calculated for different catalyst systems. Catalyst
0.00248
ZnO
AZ-5
AZ-10
AZ-15
AZ-20
0.00173
0.00264
0.00313
0.00365
0.00231
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an
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cr
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k (min−1)
P25
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Fig.1 XRD patterns of (a) porous flower-like ZnO nanostructure, (b) AZ-5, (c) AZ-10,
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(d) AZ-15, (e) AZ-20.
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Fig. 2 SEM images of (a, b) the porous flower-like ZnO; (c, d) AZ-5 heterostructure
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composites.
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Fig. 3 TEM images of (a, b) porous flower-like Ag/ZnO composites; (c) the elemental
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spectrum of AZ-5 composites revealed by EDS.
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Fig. 4 XPS spectra of the porous flower Ag/ZnO (a) XPS full spectra of the samples,
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(b) Ag 3d spectra,(c) Zn 2p spectra, (d) fit spectra of O 1s for Ag/ZnO.
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Fig. 5 N2 adsorption/desorption isotherms of ZnO and AZ-15 and BJH pore diameter
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distribution of ZnO and AZ-15.
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Fig. 6 UV–vis diffuse reflectance spectra of porous 3D (a) ZnO, (b) AZ-5, (c) AZ-20,
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(d) AZ-10, (e) AZ-15.
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Fig. 7 PL spectra of Ag/ZnO composites prepared with various Ag contents.
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Fig. 8 Photodegradation curves of RhB by different catalysts under UV-light
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irradiation.
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Fig. 9 Photodegradation curves of RhB by different catalysts under visible-light
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irradiation.
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Fig. 10 UV-light-induced charge separation mechanism of the porous flower-like
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Ag/ZnO and photocatalytic mechanism.
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Graphical Abstract Porous 3D flower-like Ag/ZnO heterostructural composites were fabricated by hydrothermal and photochemical deposition methods. Such the unique porous 3D
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structure of Ag/ZnO composites displays excellent photocatalytic activity on
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Rhodamine B. And the composite of Ag/ZnO is a promising candidate material for
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future treatment of contaminated water.
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Highlights 1. Unique porous 3D flower-like Ag/ZnO heterostructural composites was successfully synthesized by hydrothermal and photochemical deposition methods.
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2. In contrast to the conventional methods for preparing porous and flower-like
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materials, in this paper, no pore-directing reagents or surfactants are used during the
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fabrication of porous 3D Ag/ZnO structures which can avoid material post-processing and suffering from contamination.
the
Ag/ZnO
heterostructural
composite
exhibited
superior
photocatalytic
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performance.
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3. Compared with commercial TiO2 (Degussa, P25), as-prepared ZnO and Ag/ZnO,
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S0169-4332(15)00141-5 http://dx.doi.org/doi:10.1016/j.apsusc.2015.01.116 APSUSC 29553
To appear in:
APSUSC
Received date: Revised date: Accepted date:
7-11-2014 12-1-2015 17-1-2015
Please cite this article as: Y. Liang, N. Guo, L. Li, R. Li, G. Ji, S. Gan, Fabrication of porous 3D flower-like Ag/ZnO heterostructure composites with enhanced photocatalytic performance, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.01.116 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of porous 3D flower-like Ag/ZnO heterostructure composites with enhanced photocatalytic performance 1
College of Chemistry, Jilin University, Changchun 130026, P.R. China
*Corresponding authors: Guijuan Ji; Shucai Gan
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E-mail address: [email protected] (G. Ji); [email protected] (S. Gan).
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Yimai Liang1, Na Guo1, Linlin Li1, Ruiqing Li1, Guijuan Ji1*, Shucai Gan1*
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Tel: +86 431 88502259.
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Abstract Porous 3D flower-like Ag/ZnO heterostructural composites were fabricated by hydrothermal
and
photochemical
deposition
methods,
without
using
any
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pore-directing reagents and surfactants. The obtained samples were characterized by
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XRD, SEM, TEM, XPS, BJH, DRS, PL spectrum. The experiment results show that,
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the silver nanoparticles successfully load on the surface of assembled ZnO flowers. The TEM and SEM morphologies demonstrated unique porous 3D flower-like
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structure of Ag/ZnO. Such special structure makes larger surface area and more active sites exposed during the reaction, facilitating the transportation of reactants and
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products and increasing the reaction rate. The photocatalytic degradation experiments under UV irradiation using Rhodamine B (RhB) as a model dye were executed. The
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relative results demonstrate that the photocatalytic activity of Ag/ZnO is obviously improved compared with the pure ZnO and the commercial TiO2 (Degussa, P25), the
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AZ-15 sample has the highest photocatalytic activity. The Ag/ZnO heterostructure composites promoted the separation of photo-induced electrons and holes, which was proved by Photoluminescence spectra (PL). Keywords: ZnO, Ag/ZnO, porous, 3D heterostructure, Photocatalytic properties 1. Introduction
During recent decades, the environmental problem has become a block for economic development and human health. Choosing green materials to handle contaminants is particularly important. Semiconductors, especially metal-oxide semiconductors materials, have become more and more attractive due to unusual
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optical, electrical and catalytic properties [1, 2]. Their application in photocatalytic fields has received intensive attention because the technique can eliminate pollutant with a high efficiency without causing additional damage to the environment [3, 4].
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ZnO, as an important wide-direct band gap (3.37 eV) metal-oxide semiconductor, is
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promising photocatalyst owing to its high catalytic efficiency, broader ultraviolet (UV)
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light absorption, lower cost than TiO2, and environmental sustainability [5]. In addition, the ZnO photocatalyst can be easily manipulated with desirable
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microstructures, which are proved to be important factors affecting the photocatalytic activities. Especially, 3D porous structures of ZnO show superior photocatalytic
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properties as compared with various low-dimensional ZnO nanostructures. In terms of morphological structure, the use of 3D hierarchically assembled nanoflakes or
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nanoplatelets structures is likely to improve structural–mechanical stability and dispersity in comparison to 1D vertically aligned nanowires and 0D nanoparticles
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structures [6].
However, the rapid recombination of photo-induced hole–electron pairs has
become principle factor affecting photocatalytic efficiency of ZnO photocatalyst [7]. Some attempts use electron scavenging agents, such as noble metals [8-10], metal oxides [11, 12], or carbon materials [13, 14], to reduce electron–hole recombination rate. Among them, ZnO–noble metal type heterostructures are one of the most promising hybrid materials. In principle, noble metal can serve as a scavenger for photogenerated electrons, promoting interfacial electron–hole separation in the photocatalytic process, thereby increasing the number of “live” photocharges and
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improving the photocatalytic activity [15]. Ag is the cheapest noble metal, which should be beneficial to the practical application of Ag-based materials. In recent years, the 3D Ag/ZnO heterostructural composites with different morphologies have been
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successfully synthesized using different methods. For example, Han et al prepared
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Ag/ZnO flower heterostructures on indium doped tin oxide (ITO) glass via a
Ag/ZnO
heterostructural
composites
by
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photoreduction method [16]. Zhang et al reported the fabrication of flower-like hydrothermal
combine
with
an
microwave-assisted polyol method [17]. Although there are some reports about flower-like Ag/ZnO to be successfully synthesized, the above-cited methods require
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complex and expensive equipments, toxic reagents, severe synthesis conditions or high synthesis temperature. Thus, it is a key challenge to synthesize 3D flower-like
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Ag/ZnO with an environmentally friendly method. Moreover, to the best of our knowledge, there is few literature reported about the architectures of porous 3D
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flower-like Ag/ZnO composites.
In the present work, we have been successfully prepared porous 3D flower-like
Ag/ZnO by hydrothermal and photochemical depositing methods. In contrast to the conventional methods for preparing porous and flower-like materials, in this paper, no pore-directing reagents or surfactants are used during the fabrication of porous 3D Ag/ZnO structures, which can avoid material post-processing and suffering from contamination [18, 19]. The porous structure of the Ag/ZnO photocatalyst is helpful for light-harvesting and mass transportation of organic molecules in the photochemical reaction of organic degradation [20]. A systematic study is presented
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for detailed characterization of microstructure, phase, composition, and optical property of the samples. Finally, the photocatalytic activity of as-prepared composites is compared, and a possible mechanism of photocatalysis is also discussed and
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proposed.
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2. Experimental
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2.1. Materials
Zinc nitrate hexahydrate (Zn(NO3)2 6H2O), urea (CO(NH2)2) and silver nitrate
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(AgNO3) were purchased from Tianjin Chemical Reagent Research Company. All chemicals were of analytical grade and were used directly without further purification.
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Distilled water was used in all experiments.
2.2 Synthesis of porous 3D flower-like ZnO
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1mmol Zn(NO3)2 6H2O and 2 mmol urea were dissolved in 70 mL of distilled water. After being stirred for 0.5h, the mixture was then transferred to a 50 mL
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Teflon-line autoclave and sealed. The autoclave was heated to 95°C and maintained at this temperature for 6h. After naturally cooling down to room temperature, the resulting precipitates were collected by centrifugation, washed three times with ethanol and distilled water and then dried at 70°C in air for 12 h. The final porous flower-like ZnO were retrieved through a heat treatment of the precursors at 300°C in air for 2 h with a heating rate of 1 °C/min. 2.3. Fabrication of porous 3D flower-like Ag/ZnO heterostructure composites Porous
3D
flower-like
Ag/ZnO
composites
were
synthesized
through
photodeposition method [21, 22]. In detail, 0.3 g of the flower-like ZnO powder was
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dispersed in 300 mL AgNO3 aqueous solution, and stirring for 0.5 h to achieve the adsorption of Ag+ ions onto the surface of flower-like ZnO. The obtained suspension was irradiated for 2 h under radiation with continuously stirring by employing a
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high-pressure Hg UV lamp (GGZ175, 175 W) to reduce the adsorbed Ag+ to Ag
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nanoparticles, subsequently. Then, the precipitates were centrifuged, washed with
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distilled water to remove the residual Ag+. The products were dried in vacuum at 60°C for 12 h. A series of porous flower-like Ag/ZnO composites were obtained by a
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similar procedure, except for different amount of AgNO3. The molar ratio of AgNO3/Zn(NO3)2 was changed in the range from 0 to 20% and the products were
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denoted as ZnO, AZ-5, AZ-10, AZ-15, and AZ-20 respectively, in which the last
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2.4. Characterization
d
number corresponds to the molar ratio of AgNO3/Zn(NO3)2.
X-ray diffraction (XRD) analysis of porous flower-like ZnO and Ag/ZnO were
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performed using a D/Max-IIIC (Rigaku, Japan) with Cu Ka radiation. The
morphology and dimension of the products were characterized by S-4800 field emission scanning electron microscope (FE-SEM, Hitachi, Japan) with an acceleration voltage of 3 kV. Further detailed structural analysis of individual particles was carried out by using a Hitachi 8100 transmission electron microscope (TEM, Hitachi, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI-5000CESCA system with Mg K radiation (hr = 1253.6 eV). The X-ray anode was run at 250 W, and the high voltage was kept at 14.0 kV with a detection angle at 540. All the binding energies were calibrated by using the
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containment carbon (C 1s = 284.6 eV). The Brunauer-Emmett-Teller (BET) surface area of the powders was analyzed by nitrogen adsorption on a Micromeritics ASAP 2020 nitrogen adsorption apparatus (U.S.A.). The sample was degassed in vacuum at
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150°C for 7h before nitrogen adsorption measurement. The BET surface area was
cr
determined by a multipoint BET method using the adsorption data in the relative
determine
the
pore size
distributions for
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pressure (P/P0) range of 0.01-0.99. Adsorption branches of the isotherms were used to the
samples
studied
via
the
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Barrett-Joyner-Halenda (BJH) method. The volume of nitrogen adsorbed at the relative pressure (P/Po) of 0.99 was used to determine the pore volume. Differential
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scanning calorimetry-thermogravimetric analyzer (DSC/TGA 1600 LF, METTLER TOLEDO, Switzerland) up to 800 °C was performed on the sample at a heating rate of
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d
10 °C min−1 while N2 gas flow rate of 60 ml min−1. 2.5 Photocatalytic experiments
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The photocatalytic performance of Ag/ZnO heterostructure was evaluated by using
RhB as presentative dye pollutant. A high-pressure Hg UV lamp (GGZ175, 175 W) with a maximum emission at 365 nm served as the UV light resource. A 500W Xenon lamp (CEL-HXF300) was used as the visible light source with a 400 nm cut-off filter. In a typical procedure, the 0.04 g catalyst (Ag/ZnO, Degussa P25 and ZnO) was dispersed into 20 mL aqueous solution of RhB (10 mg L-1). After stirring in the dark for 0.5 h, the suspensions were placed under light irradiation. During the photoreaction, the samples were collected at regular intervals and centrifuged to remove the catalyst. The concentration of target organic solution before and after
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degradation was measured by using a UV–vis spectrometer (UV-2550, Shimadzu, Japan). 3. Results and discussion
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3.1 Morphological and structural analysis
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The structural properties of the synthetic samples were analyzed by X-ray
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diffraction (XRD). Fig. 1 shows the XRD patterns for the porous flower-like ZnO and Ag/ZnO composites with different Ag content. Three main distinct peaks at 31.78°,
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34.42°, and 36.28° are observed in above patterns, which are indexed to the (100), (002), and (101) diffractions of the wurtzite ZnO (JCPDS no. 36-1451). From Fig.1, it
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can be seen that Ag/ZnO composite samples had four major peaks at 38.14°, 44.26°, 64.42°, 77.44°, which were readily assigned to the (111), (200), (220), and (311)
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planes of face-center-cubic (fcc) structure of silver (JCPDS no. 04-783), respectively. Additionally, no other peaks and remarkable shifts with increase amount of Ag, which
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indicates that no related solid solution of ZnO and Ag is formed or lattice expansion and shrinkage of Ag/ZnO should be negligible. In order to characterize the morphologies of the as-prepared ZnO and Ag/ZnO
composites, SEM and TEM were carried out. As is shown in Fig. 2a, ZnO has a three-dimension (3D) flower-like morphology and a few microns in size. A detailed observation of a single ZnO is shown Fig. 2b, those flower-like architectures are formed by a lot of self-assembled porous nanoplates and the thickness of most of nanoplates is 10-19nm. In addition, we can obviously observe that there are numerous pores in the nanosheets in the inset of 2b. Such a porous network structure can
Page 8 of 40
potentially increase the accessible surface area for enhanced reactant diffusivity, which is highly favourable for catalytic applications. Fig. 2c and d shows the SEM
morphology has not been changed upon modification with Ag.
ip t
image of a typical Ag/ZnO flower-like structure. It can be seen that the ZnO
cr
Fig. 3a shows the TEM image of a typical Ag/ZnO flower-like structure,
us
confirming the results obtained by SEM. It can be found that the Ag/ZnO particles remain flower-like structure and a great amount of spherical Ag particles with
an
diameter ranging from 90 to 130 nm are observed on the surfaces of ZnO flowers, suggesting that the structure of ZnO is maintained after the formation of Ag
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nanoparticles. The porous feature of Ag/ZnO nanosheets is further confirmed by the corresponding TEM image (Fig. 3b) of individual ZnO porous nanosheets. It is
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apparent that the mesopores with irregular shape are of 10–50 nm in size. The large amount of the nanopores in the nanostrips should be beneficial to enhancing the
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specific surface area of the products. The EDS analysis, as seen in Fig. 3c, confirms that these nanoparticles are only composed of oxygen, zinc, and silver elements, which is consistent with the XRD results. 3.2 XPS analysis
To clarify the surface structures of porous flower-like Ag/ZnO composites, XPS
measurement was conducted on AZ-5. The survey result in Fig. 4a demonstrates all peaks are ascribed to Zn, Ag, O and C, confirming the heterostructure is composed of three elements of Zn, Ag and O. The results are in good agreement with XRD and EDS as described above. The presence of C is mainly from pump oil due to vacuum
Page 9 of 40
treatment before the XPS test [23]. High-resolution spectra of Ag, Zn and O species are shown in Fig. 4 b-d, respectively. From Fig. 4b, it can be seen that the binding energies of Ag 3d5/2 and Ag 3d3/2 for Ag/ZnO is at 367.3 and 373.4 eV, which shift
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remarkably to lower binding energy according to the corresponding values of the
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synthesized pure metallic Ag (the standard binding energies of Ag 3d5/2 and Ag 3d3/2
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for bulk Ag are about 368 and 374.2 eV, respectively) [24]. This phenomenon is similar to the results obtained from ZnO/Ag nanorods heterostructures and Ag/ZnO
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hollow sphere composites [25, 26]. The binding energy shift of Ag is mainly attributed to electron transfer from metallic Ag to ZnO crystals. Since the work
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function of silver is smaller than that of ZnO, once the Ag nanoparticle and ZnO attach together, electron transfer occurs from Ag to the conduction band (CB) of ZnO
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nanocrystal. Thus, a new Fermi energy level in Ag/ZnO is formed, leading to the higher valence of Ag [27]. Because of the lower binding energy of monovalent Ag
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than that of zero-valent Ag, the shift to lower binding energies of Ag 3d5/2 and Ag
3d3/2 further verifies formation of Ag/ZnO heterostructural composites [28]. The Zn 2p3/2 and Zn 2p1/2 peak of the Ag/ZnO sample, as shown in Fig. 4c, have a value of
about 1021.3 and 1044.3 eV, which are similar to that of pure ZnO, this finding confirms that Zn exists mainly in the Zn2+ chemical state on the sample surface [29]. In Fig. 4d, the O 1s profile of Ag/ZnO can be fitted into two peaks (530.0 and 531.8 eV), indicating the presence of two different kinds of O species in the sample. The peaks are associated with the lattice oxygen of ZnO and chemisorbed oxygen of the surface hydroxyls, respectively [30]. Herein, it is worth mentioning that the surface
Page 10 of 40
hydroxyls can produce primary active hydroxyl radicals, which are capable of trapping photoinduced electrons and holes. Thus, the surface hydroxyls are very important for photocatalysis [31].
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3.3 BET analysis
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The mesopores of porous flower-like Ag/ZnO composites are further confirmed
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by the responding plot of the pore size distribution is determined by using the Barrett-Joyner-Halenda (BJH) method from the desorption branch of the isotherm.
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Full nitrogen sorption isotherms of the porous flower-like ZnO and AZ-15 are shown in Fig. 5. It can be noted that most of the pores possess diameters in the range of
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10–50 nm with the peak zenith of ZnO and AZ-15 appearing around 22.3 and 17.8 nm (see the inset of Fig. 5). Pore size distribution curves confirm the mesoporous nature
d
of porous flower-like ZnO and Ag/ZnO heterostructures. The specific surface area
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calculated by BET method is 49.0 m2 /g for porous flower-like ZnO and 34.5 m2/g for
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AZ-15. The results are consistent with a previous report [32]. Table 1 summarizes the calculating BET surface areas of porous flower-like ZnO and Ag/ZnO heterostructures and comparion with the relevant literature reports [33-36]. 3.4 UV–vis diffuse reflectance and Photoluminescence spectrum The UV–vis spectra of porous flower ZnO and Ag/ZnO composites with
variable contents are shown in Fig. 6. The absorption peaks at UV region are assigned to the absorption of ZnO. The pure ZnO maximum absorption edge is located at around 372 nm. As shown in Fig. 6b-d the absorption of the porous flower-like Ag/ZnO composites shows the enhanced absorption at UV region. By comparing ZnO
Page 11 of 40
with Ag/ZnO, it can still be found that the performance of UV–vis absorption is apparently enhanced as Ag is doped. In addition, there are two absorption band near 490 nm and 670 nm for all the Ag/ZnO catalysts besides the absorption edge of ZnO,
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which can be assigned to the characteristic absorption of surface plasmon resonance
cr
(SPR) caused by nanometallic Ag [37]. The surface plasmon resonance effect of Ag
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particles can increase the absorption capability for visible light, which induced the visible light responsive photocatalytic activity for ZnO photocatalyst [38]. Compared
an
with the porous ZnO, the absorption spectrum of Ag/ZnO reveals an expansion from UV to visible area, which indicates potential application of porous 3D Ag/ZnO
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heterostructure composites as sunlight-derived photocatalyst.
Notably, it has been duly established that the photoluminescence spectra originate
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from the direct recombination of photoexcited electrons and holes [39]. The Photoluminescence (PL) spectra (excitation at 325 nm) of as-prepared ZnO and
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Ag/ZnO with different Ag contents composites are shown in Fig. 7. A UV peak centered at about 380 nm is observed, which is assigned to the recombination of electron–hole pairs in the semiconductor through an exciton–exciton collision process. Meanwhile, it is worth to note that the intensity of PL peak varies with the introduction of Ag, inferring that the Ag can change the recombination of electron–hole pairs. On the one hand, the PL spectra of Ag/ZnO shows a decrease in the intensity of UV emission peak, which results from electron transfer from ZnO to Ag on the photocatalyst interface. Moreover, it can be seen clearly that the sample AZ-15 shows the minimum PL intensity [40]. This phenomenon can be interpreted as
Page 12 of 40
that, with the increase of Ag, more metal sites are formed and available to trap electrons, leading to a corresponding increase in separation effects for the photoinduced electrons and holes, which would enhance the photocatalytic properties
ip t
of the as-obtained heterostructures [41]. On the other hand, when the Ag content
cr
exceeds certain value (as sample AZ-20), the PL intensity increases again. This can be
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attributed to absorption or reflection of emission at the Ag/ZnO interface, which is mainly induced by the strong surface plasmon absorption of Ag particles [42].
an
According to the above analysis, it is that Ag modification can make the intensity of PL decrease. The weaker is PL response, the better is the separation efficiency of
performance of Ag/ZnO composites.
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photoinduced electron–hole pairs, and the higher will be the photocatalytic
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3.5 Photocatalytic performance
The photocatalytic activity of Ag/ZnO composites was evaluated by taking RhB as
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the target dye pollutant. Under UV light irradiation, the photocatalytic results of RhB in the presence of different catalysts (Ag/ZnO, ZnO and P25) are all shown in Fig. 8. Under dark conditions, the concentration of RhB has no obvious change for long time in the presence of the catalysts. Similar result was obtained when reaction was performed under UV-light irradiation but without using any catalysts. The results indicate that both the light and ZnO are necessary for effective photodegradation of RhB dye. As shown in Fig. 5, the photodegradation efficiency of RhB is about 27%, 21%, 56%, 63% ,79% and 41% for P25, ZnO, AZ-5, AZ-10, AZ-15 and AZ-20, respectively, when the reaction was performed under UV-light for 120 min. It can be
Page 13 of 40
seen that the Ag/ZnO composite is a superior photocatalyst to Degussa P25 for photodegradation of the RhB. However, the pure ZnO particles exhibit a lower activity than that of Degussa P25. The photodegradation of RhB can be considered as
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a pseudo-first-order reaction, the rate expression is given by equation: ln(C/C0) = -kt
cr
[43]. The apparent rate constant k was listed in Table 2. Based on the results, one can
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see that k value for all Ag/ZnO samples is higher than Degussa P25 and pure ZnO. The AZ-15 sample shows the highest catalytic activity with rate constant k = 0.0365
an
min−1 that is about 2 times higher than that of pure ZnO (0.00173 min−1). These results show that the photocatalytic activity of ZnO can be obviously improved in the
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presence of Ag NPs. They also indicate that an appreciable amount of Ag is necessary to enhance the photocatalytic performance.
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d
It can be seen that the photocatalytic activities of these samples vary in the following order: ZnO< P25< AZ-20< AZ-5 < AZ-10< AZ-15. We can note that the
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activity of Ag/ZnO composites firstly increased and then decreased with the increase of Ag amount in the composites. The AZ-15 exhibits the highest photodegradation rate. It consistent with the results of Fig.7, the PL studies indicate that the deposition of appropriate amount of Ag on ZnO surface is necessary to inhibit the recombination of photo-induced electron–hole pairs [44]. It is well known that the strong ultraviolet emission corresponds to the high electron–hole recombination. That is to say, the AZ-15 shows much higher electron–hole segregation efficiency than other as-synthesized catalysts, which is an important reason for its much better photocatalytic activity. However, the photocatalytical activity of Ag/ZnO remarkably
Page 14 of 40
decreases when the Ag/Zn atomic ratio is increased to 20%. This phenomenon can be interpreted as follows: (1) after the Ag content exceeds the optimum value, the active sites on the surface of Ag/ZnO will decrease and the UV light adsorption by ZnO can
ip t
be partly shielded, decreasing the utilizing efficiency of photoelectrons. (2) Ag can
cr
also reversibly act as charge carrier recombination centers, which is caused by the
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electrostatic attraction of negatively charged Ag and positively charged holes when Ag is excessive [45]. The over accumulations of electrons on metal surface could
an
attract the photogenerated holes to the metal sites, and promote the recombination of charge carriers [46]. In other words, the over accumulations of negatively charged Ag
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could increase the possibility of hole capture, reduce the efficiency of charge separation, and suppress the photocatalytic activity of Ag/ZnO.
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d
Based on above analysis, the effect of Ag modification on the photocatalytic performance ZnO could be explained by followed points: (1) Ag NPs could inhibit the
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recombination of photoinduced carriers of ZnO, thus the photocatalytic performance of ZnO could be improved by Ag modification; (2) excessive Ag can shield the UV light adsorption of ZnO, thus decrease the photon utilizing efficiency. As results, the photocatalytic activity of the catalyst is therefore depressed as more Ag was used. Present results about effect of Ag content on photocatalytic activity are consistent with previous reports [44, 47]. Further investigation on photocatalytic activity of porous 3D flower-like Ag/ZnO under visible light (λ>400 nm) was also carried out. Fig. 9 shows the
photodegradation curves of RhB by different catalysts under visible-light irradiation.
Page 15 of 40
It can be seen that the pure ZnO and P25 have weak visible light photocatalytic activity due to ZnO and TiO2 with wide band gap energy. Comparing with the pure ZnO, all Ag/ZnO samples exhibit much higher photocatalytic activities. As discussed
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in the diffuse reflectance spectroscopy (Section 3.4), pure ZnO has no absorption in
cr
the visible region, therefore, it has no photocatalytic activity under the visible light,
us
but, by depositing Ag nanoparticles RhB photodegradation improved under similar condition. Under visible light irradiation, ZnO cannot be excited due to its wide
an
bandgap energy (3.37 eV). However, due to the surface plasmon resonance of Ag nanoparticles, most photogenerated electrons may be excited from the surfaces of Ag
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nanoparticles to the conduction band of ZnO and then diffuse to the surrounding medium to promote photocatalysis. This provides a facile and efficient pathway for
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d
the photocatalytic degradation of RhB in the visible-light region [48]. 3.6 Photocatalytic mechanism
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Scheme diagram of the photocatalytic reaction on Ag/ZnO heterostructure is
presented in Fig. 10. Under irradiated under UV light, photon energy is higher than or equal to the band gap of ZnO crystals and electrons (e−) in the valence band (VB) will be excited to the CB with simultaneous generation of the same amount of holes (h+) in
the VB(Eq. (1) ). Due to the higher CB of ZnO than the new equilibrium Fermi energy level of Ag/ZnO, some electrons are injected into Ag nanoparticles quickly. At the same time, a Schottky barrier formed at the semiconductor–metal interface, and the holes can remain in the semiconductor surfaces [49]. Hence, silver, acting as electron sinks, not only reduce the recombination of photoinduced electrons and holes
Page 16 of 40
but also prolong the lifetime of photogenerated pairs. The electron will continually transfer until the overall Fermi level of the metal-modified ZnO system shifts toward more negative potential and ultimately equilibrates with that of ZnO. Subsequently,
ip t
the transferred electrons can be easily trapped by the adsorbed O2 molecules to
cr
produce •O2- radical (Eq. (2) ). While, photoinduced holes in ZnO can be also readily
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scavenged by the immanence H2O molecules to yield •OH radicals (Eq. (3) ). The latter is an extremely strong oxidant for degeneration of organic chemicals, which can
follows [50]. ZnO h ZnO(e- h )
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-
h vb OH OH
(2)
(3)
d
-
e cb O 2 O 2
(1)
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be destroyed into CO2 and H2O activities (Eq. (4) ). This process can be proposed as
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4. Conclusions
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OH RhB degradation product (4)
In summary, we have been successfully fabricated porous 3D flower-like Ag/ZnO
by hydrothermal and photochemical depositing methods. Such special structure with an open and porous surface layer, which be benefit for facilitate the diffusion and mass transportation of organic molecules and oxygen species during the photochemical reaction. The photocatalytic degradation experiments demonstrate that, the as-prepared Ag/ZnO samples exhibited superior photocatalytic performance, which was attributed to the unique porous 3D structure of Ag/ZnO and effectively separation of photo-generated charge on flower-like ZnO by employing Ag
Page 17 of 40
nanoparticles as a conductor. In addition, the AZ-5 composite showed the highest photochemical activity. On the basis of the structural characterizations and photocatalytic results, the effect of Ag nanoparticles on the photocatalytic
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performance of porous 3D flower-like ZnO composites can be summarized as follows:
cr
(1) Ag deposit acted as electron sinks to enhance the separation of photoexcited
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electrons from holes; (2) Ag particles also act as recombination centers at high silver deposition which leads to the decrease of the material’s photocatalytic activity. The
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study revealed that the prepared composite exhibits a high potential for treatment of contaminated water applications.
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Acknowledgments
This work was supported by Mineral and Ore Resources Comprehensive Utilization
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of Advanced Technology Popularization and Practical Research (MORCUATPPR) founded by the China Geological Survey (grant no. 12120113088300). It was also
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supported by Key Technology and Equipment of Efficient Utilization of Oil Shale Resources (no. OSR-05) and the National Science and Technology Major Projects (no. 2008ZX05018). References
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photocatalytic activity, Applied Surface Science 256 (2010) 7125–7130. [49] X. Chen and S. S. Mao, Titanium Dioxide Nanomaterials:Synthesis, Properties, Modifications, and Applications, Chem. Rev, 107 (2007) 2891–2959. [50] Gao, S. Y.; Jia, X. X.; Yang, S. X.; Li, Z. D.; Jiang, K, Hierarchical Ag/ZnO micro/nanostructure: Green synthesis and enhanced photocatalytic performance, J. Solid State Chem. 184 (2011)764−769.
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Figure captions Fig.1 XRD patterns of (a) porous flower-like ZnO nanostructure, (b) AZ-5, (c) AZ-10, (d) AZ-15, (e) AZ-20.
ip t
Fig. 2 SEM images of (a, b) the porous flower-like ZnO; (c, d) AZ-5 heterostructure
cr
composites.
spectrum of AZ-5 composites revealed by EDS.
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Fig. 3 TEM images of (a, b) porous flower-like Ag/ZnO composites; (c) the elemental
an
Fig. 4 XPS spectra of the porous flower Ag/ZnO (a) XPS full spectra of the samples, (b) Ag 3d spectra,(c) Zn 2p spectra, (d) fit spectra of O 1s for Ag/ZnO.
distribution of ZnO and AZ-15.
M
Fig. 5 N2 adsorption/desorption isotherms of ZnO and AZ-15 and BJH pore diameter
(d) AZ-10, (e) AZ-15.
te
d
Fig. 6 UV–vis diffuse reflectance spectra of porous 3D (a) ZnO, (b) AZ-5, (c) AZ-20,
Ac ce p
Fig. 7 PL spectra of Ag/ZnO composites prepared with various Ag contents. Fig. 8 Photodegradation curves of RhB by different catalysts under UV-light
irradiation.
Fig. 9 Photodegradation curves of RhB by different catalysts under visible-light
irradiation.
Fig. 10 UV-light-induced charge separation mechanism of the porous flower-like Ag/ZnO and photocatalytic mechanism.
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Table 1 BET surface areas of porous flower-like ZnO and Ag/ZnO heterostructures
BET Surface area ± 0.1 (m2/g) 49.0
Porous flower-like Ag/ZnO
34.5
nanoplates ZnO [32]
15.5
0nanospheres ZnO [33]
20.8
Commercial ZnO powder [34]
3.6
Ag/ZnO particles [35]
3.5
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te
d
M
an
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porous flower-like ZnO
cr
Photocatalyst
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prepared in this paper and relevant literature reports.
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Table 2 The apparent rate constant K calculated for different catalyst systems. Catalyst
0.00248
ZnO
AZ-5
AZ-10
AZ-15
AZ-20
0.00173
0.00264
0.00313
0.00365
0.00231
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te
d
M
an
us
cr
ip t
k (min−1)
P25
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ip t cr us
Fig.1 XRD patterns of (a) porous flower-like ZnO nanostructure, (b) AZ-5, (c) AZ-10,
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te
d
M
an
(d) AZ-15, (e) AZ-20.
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ip t cr us an
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Fig. 2 SEM images of (a, b) the porous flower-like ZnO; (c, d) AZ-5 heterostructure
Ac ce p
te
d
composites.
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Fig. 3 TEM images of (a, b) porous flower-like Ag/ZnO composites; (c) the elemental
Ac ce p
te
d
spectrum of AZ-5 composites revealed by EDS.
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ip t cr us an
M
Fig. 4 XPS spectra of the porous flower Ag/ZnO (a) XPS full spectra of the samples,
Ac ce p
te
d
(b) Ag 3d spectra,(c) Zn 2p spectra, (d) fit spectra of O 1s for Ag/ZnO.
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ip t cr us
Fig. 5 N2 adsorption/desorption isotherms of ZnO and AZ-15 and BJH pore diameter
Ac ce p
te
d
M
an
distribution of ZnO and AZ-15.
Page 33 of 40
ip t cr us
Fig. 6 UV–vis diffuse reflectance spectra of porous 3D (a) ZnO, (b) AZ-5, (c) AZ-20,
Ac ce p
te
d
M
an
(d) AZ-10, (e) AZ-15.
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ip t cr us
Ac ce p
te
d
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an
Fig. 7 PL spectra of Ag/ZnO composites prepared with various Ag contents.
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ip t cr us
Fig. 8 Photodegradation curves of RhB by different catalysts under UV-light
Ac ce p
te
d
M
an
irradiation.
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us
Fig. 9 Photodegradation curves of RhB by different catalysts under visible-light
Ac ce p
te
d
M
an
irradiation.
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Fig. 10 UV-light-induced charge separation mechanism of the porous flower-like
Ac ce p
te
d
M
an
Ag/ZnO and photocatalytic mechanism.
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Graphical Abstract Porous 3D flower-like Ag/ZnO heterostructural composites were fabricated by hydrothermal and photochemical deposition methods. Such the unique porous 3D
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structure of Ag/ZnO composites displays excellent photocatalytic activity on
cr
Rhodamine B. And the composite of Ag/ZnO is a promising candidate material for
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te
d
M
an
us
future treatment of contaminated water.
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Highlights 1. Unique porous 3D flower-like Ag/ZnO heterostructural composites was successfully synthesized by hydrothermal and photochemical deposition methods.
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2. In contrast to the conventional methods for preparing porous and flower-like
cr
materials, in this paper, no pore-directing reagents or surfactants are used during the
us
fabrication of porous 3D Ag/ZnO structures which can avoid material post-processing and suffering from contamination.
the
Ag/ZnO
heterostructural
composite
exhibited
superior
photocatalytic
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te
d
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performance.
an
3. Compared with commercial TiO2 (Degussa, P25), as-prepared ZnO and Ag/ZnO,
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